1. Field of the Invention
The present invention relates to a parallel combinational logic system having a shortened carry run for two binarily-coded operands upon consideration of an input carry, comprising adders and a decision logic connected in groups and time stages for determining conditional carries that are assigned to said groups.
2. Description of the Prior Art
The logic operation of two binary numbers can be reduced to the addition of two operands for all four basic arithmetic operations, as is well known in the art. Parallel processing is thereby preferred over serial processing when a high-operating speed is desired. Given the former calculating method, the bit sequences of the two operands are sub-divided into groups, for example digit-wise or byte-wise, and are simultaneously combined with one another in a correct digit position.
A complete simultaneity of the combination, however, is opposed by the processing of the carries which must, under given conditions, be carried from one group into the next. Their throughput time defines the processing time of a parallel adder.
A method for shortening the throughput time of the carries is disclosed in the article by J. Sklanski entitled "Conditional-Sum-Addition-Logic" published in IRE Transactions and Electronic Computers, June 1980, pp. 226-231. This addition mode is based on the calculation of "conditional" sums and carries. Alternative sums for individual groups of bit sequences are thereby identified in successive time intervals under the assumption that all possible carries occur. As soon as the actual value of an input carry for a group is determined, the results or, respectively, intermediate sums and intermediate carries already calculated for this carry, are selected in the next processing stage and further calculation is based thereon. Consequently, at least a part of the already-calculated bit sequences must thereby be adapted or, respectively, newly added up, in each of the processing stages. The circuit expense required for the implementation of this method expands to an ever-increasing degree with an increasing processing scope of the operands.